On 21/02/2020 17:50, Jens Axboe wrote: > On 2/21/20 6:51 AM, Pavel Begunkov wrote: >> On 20/02/2020 23:31, Jens Axboe wrote: >>> For poll requests, it's not uncommon to link a read (or write) after >>> the poll to execute immediately after the file is marked as ready. >>> Since the poll completion is called inside the waitqueue wake up handler, >>> we have to punt that linked request to async context. This slows down >>> the processing, and actually means it's faster to not use a link for this >>> use case. >>> >>> We also run into problems if the completion_lock is contended, as we're >>> doing a different lock ordering than the issue side is. Hence we have >>> to do trylock for completion, and if that fails, go async. Poll removal >>> needs to go async as well, for the same reason. >>> >>> eventfd notification needs special case as well, to avoid stack blowing >>> recursion or deadlocks. >>> >>> These are all deficiencies that were inherited from the aio poll >>> implementation, but I think we can do better. When a poll completes, >>> simply queue it up in the task poll list. When the task completes the >>> list, we can run dependent links inline as well. This means we never >>> have to go async, and we can remove a bunch of code associated with >>> that, and optimizations to try and make that run faster. The diffstat >>> speaks for itself. >> >> So, it piggybacks request execution onto a random task, that happens >> to complete a poll. Did I get it right? >> >> I can't find where it setting right mm, creds, etc., or why it have >> them already. > > Not a random task, the very task that initially tried to do the receive > (or whatever the operation may be). Hence there's no need to set > mm/creds/whatever, we're still running in the context of the original > task once we retry the operation after the poll signals readiness. Got it. Then, it may happen in the future after returning from __io_arm_poll_handler() and io_uring_enter(). And by that time io_submit_sqes() should have already restored creds (i.e. personality stuff) on the way back. This might be a problem. BTW, Is it by design, that all requests of a link use personality creds specified in the head's sqe? -- Pavel Begunkov
